Thin film technology has been used for a number of years to control the transmittance and/or reflectance of coated surfaces. By controlling the thickness and index of refraction of each film in a stack or array of thin films constituting a coating one can tailor the reflective and transitive characteristics of the coating. The design of such films and the principals used to achieve such results are well known to those skilled in the art as is the general knowledge of how to deposit the various thin films on various substrates.
Various classes of coatings exist based not only upon the materials used to form said coatings, but based upon spectral reflectance characteristics as well. For instance, reflective coatings have long been available which transmit in the infrared region and reflect all or most of the visible portion of the spectrum. Such coatings are known generically as "cold mirrors". Alternately, coatings are also known which transmit the infrared portion of the spectrum and reflect only a a fraction of the visible spectrum. These coatings are generically known as "color correcting cold mirror" coatings.
For many commercial applications such coatings are provided on glass substrates in the form of reflector blanks or filter blanks. However, recently it has become desirable to form filters and reflectors out of plastic materials. See, for instance, U.S. Pat. No. 4,380,794. The use of plastic reflectors poses some problems in that there are many plastic materials which have the heat stability required to be used in such applications, however, there are very few plastic materials which are sufficiently U.V. radiation stable to be suitable. U.S. Pat. No. 4,380,794 teaches that polyetherimide is sufficiently stable to provide a useful reflector material.
The present invention provides a novel optical coating which allows reflectors and other optical apparatus to be made from plastic materials which have been hereto unacceptable due to their U.V. sensitivity.